Using the radio frequency emitted by carbon monoxide molecules to measure rotation---the swirling of gas cloud around a central mass---the doppler shift of the gas coming towards versus the gas going from. It's an interesting business. I hope we can sometime get an online link to the Nature article. I don't have one.

Here's the abstract of the article just published in Nature (but not the full article):http://www.nature.com/nature/journal/vaop/ncurrent/full/nature11819.html
==quote==A black-hole mass measurement from molecular gas kinematics in NGC4526
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The masses of the supermassive black holes found in galaxy bulges are correlated with a multitude of galaxy properties1, 2, leading to suggestions that galaxies and black holes may evolve together3. The number of reliably measured black-hole masses is small, and the number of methods for measuring them is limited4, holding back attempts to understand this co-evolution. Directly measuring black-hole masses is currently possible with stellar kinematics (in early-type galaxies), ionized-gas kinematics (in some spiral and early-type galaxies5, 6, 7) and in rare objects that have central maser emission8. Here we report that by modelling the effect of a black hole on the kinematics of molecular gas it is possible to fit interferometric observations of CO emission and thereby accurately estimate black-hole masses. We study the dynamics of the gas in the early-type galaxy NGC 4526, and obtain a best fit that requires the presence of a central dark object of 4.5+4.2-3.1 × 108 solar masses (3σ confidence limit). With the next-generation millimetre-wavelength interferometers these observations could be reproduced in galaxies out to 75 megaparsecs in less than 5 hours of observing time. The use of molecular gas as a kinematic tracer should thus allow one to estimate black-hole masses in hundreds of galaxies in the local Universe, many more than are accessible with current techniques.
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Preprint just posted half an hour ago! Now we have free access, not limited to subscribers to Nature.http://arxiv.org/abs/1301.7184A black-hole mass measurement from molecular gas kinematics in NGC4526
Timothy A. Davis, Martin Bureau, Michele Cappellari, Marc Sarzi, Leo Blitz
(Submitted on 30 Jan 2013)
The masses of the supermassive black-holes found in galaxy bulges are correlated with a multitude of galaxy properties, leading to suggestions that galaxies and black-holes may evolve together. The number of reliably measured black-hole masses is small, and the number of methods for measuring them is limited, holding back attempts to understand this co-evolution. Directly measuring black-hole masses is currently possible with stellar kinematics (in early-type galaxies), ionised-gas kinematics (in some spiral and early-type galaxies) and in rare objects which have central maser emission. Here we report that by modelling the effect of a black-hole on the kinematics of molecular gas it is possible to fit interferometric observations of CO emission and thereby accurately estimate black hole masses. We study the dynamics of the gas in the early-type galaxy NGC4526, and obtain a best fit which requires the presence of a central dark-object of 4.5(+4.2-3.0)x10^8 Msun (3 sigma confidence limit). With next generation mm-interferometers (e.g. ALMA) these observations could be reproduced in galaxies out to 75 megaparsecs in less the 5 hours of observing time. The use of molecular gas as a kinematic tracer should thus allow one to estimate black-hole masses in hundreds of galaxies in the local universe, many more than accessible with current techniques.
Comments: To appear in Nature online on 30/01/2013. 3 Pages, 2 Figures (plus two pages of supplementary information)